This invention relates to a method of treating the inner surface of a plastic tube with plasma. More particularly, the present invention relates to a method of plasma-treating the inner surface of a plastic tube by use of low temperature plasma.
By virtue of their excellent properties, plastics have gained an extremely wide range of applications but their surface properties have not entirely been satisfactory; hence, a variety of surface treatment techniques have been proposed in the past. A surface treatment technique by low temperature plasma, or by so-called glow discharge, provides the advantage that the surface properties can be changed without changing the bulk properties. For this reason, it has been drawing an increasing attention as one of the excellent surface treatment techniques. For example, U.S. Pat. No. 4,265,276 proposes to treat the inner surface of a polyvinyl chloride tube by low temperature plasma so as to modify the properties of the inner surface.
However, it is difficult to initiate and sustain the low temperature plasma in a narrow and limited space such as inside a tube. Another problem lies in that since plastic tubes in general are soft and flexible, the tubes are likely to inwardly collapse once the inside of the tubes is brought into a reduced pressure or a vacuum in order to initiate the glow discharge. It has therefore been believed extremely difficult to subject the inner surface of the tubes to the low temperature plasma surface treatment.
Japanese Patent Laid-Open No. 29505/1980 proposes the following method as set forth in its claim:
"A method of treating the inner surface of a plastic tube characterized in that a plastic tube whose outer surface is exposed to the atmosphere and whose inner surface is kept at a reduced pressure is continuously moved inside an electric field or magnetic field applied by external electrodes, in order to generate low temperature plasma only inside said tube."
However, this method cannot be applied to soft tubes, or, those tubes which undergo deformation or collapse when the tube inner surface is placed into a reduced pressure or vacuum but can be applied only to those plastic tubes which are devoid of flexibility and have rigidity.
The aforementioned U.S. Pat. No. 4,265,276 is characterized in that an electrically insulating tube which is made of a dielectric material and whose internal pressure is kept at a reduced level is disposed outside the plastic tube in order to prevent the deformation and collapse of the plastic tube that occur when the internal pressure is reduced for the purpose of the plasma treatment, and the pressure inside the insulating tube as well as the pressure inside the plastic tube are individually adjusted in order to prevent the occurrence of plasma between the outside of the plastic tube and the inside of the insulating tube.
The inventors of the present invention have carried out a series of experiments of this prior art method and have found that a gas is emitted from the plastic tube or a gaseous matter is afresh formed due to discharge so that the internal pressures of the plastic tube and insulating tube must be controlled in a complicated manner. Still worse, discharge is likely to be stopped readily due to the changes of condition and at times, the discharge shifts into the insulating tube but does not occur inside the plastic tube. Similar phenomena are found occurring even due to slight deviation of matching adjustment.
Within a low pressure range for initiating the glow discharge, no significant difference is found for the discharge initiation voltage with little or no difference for the discharge sustenance voltage. For these reasons, matching adjustment must be made with an extremely high level of accuracy in order to initiate and sustain the discharge for only one of the spaces having such a little pressure difference as in this prior art method. Speaking conversely, this means that matching readily deviates depending upon a slight change of a condition such as a pressure fluctuation, and this presumably explains the phenomena the inventors of the present invention have experienced.
In any way, this is a critical problem to be solved when plasma treatment of plastic tubes is carried out on an industrial scale and hence, the prior art method is not feasible industrially.